Mesoscale networks and corresponding transitions from self-assembly of block copolymers

A series of cubic network phases was obtained from the self-assembly of a single-composition lamellae (L)-forming block copolymer (BCP) polystyrene-block-polydimethylsiloxane (PS-b-PDMS) through solution casting using a PS-selective solvent. An unusual network phase in diblock copolymers, double-primitive phase (DP) with space group of Im3¯m, can be observed. With the reduction of solvent evaporation rate for solution casting, a double-diamond phase (DD) with space group of Pn3¯m can be formed. By taking advantage of thermal annealing, order–order transitions from the DP and DD phases to a double-gyroid phase (DG) with space group of Ia3¯d can be identified. The order–order transitions from DP (hexapod network) to DD (tetrapod network), and finally to DG (trigonal planar network) are attributed to the reduction of the degree of packing frustration within the junction (node), different from the predicted Bonnet transformation from DD to DG, and finally to DP based on enthalpic consideration only. This discovery suggests a new methodology to acquire various network phases from a simple diblock system by kinetically controlling self-assembling process.

From constituted molecules to polymers, finally ordered hierarchical superstructures, self-assembled solids cover a vast area of nanostructures where the characters of building blocks direct the progress of self-assembly (1, 2). In nature, fascinating periodic network structures and morphologies from different species are appealing in nanoscience and nanotechnology due to their superior properties, especially for photonic crystal structures (3–7). For gyroid, trigonal planar network with chirality demonstrates its potential as chiropitc metamaterial (8–10). Beyond the splendid colors, networks either in macroscale or mesoscale mechanically strengthen their skeletons and protect those fragile but vital organs from impact (11, 12). Inspired by nature, biomimicking materials with mesoscale network may exceed the limitation of the intrinsic properties (13). The topology of networks could further improve their adaptability, allowing extreme deformation for energy dissipation (14). Moreover, network materials from hybridization of self-assembled block copolymers (BCPs) have been exploited to the design of mesoscale quantum metamaterials (15, 16). With the desire to acquire network textures for biomimicking nanomaterials, BCPs with immiscible constituted segments covalently joined together give the accessibility to the formation of nanonetwork morphologies via balancing enthalpic penalty from the repulsive interaction of constituted blocks and entropic penalty from the stretching of polymer chains (17–21). By taking advantage of precise synthesis procedures, it is feasible to obtain the aimed network phases from the self-assembly of BCPs such as Fddd (O⁷⁰) (22–24), gyroid (Q²¹⁴, Q²³⁰) (20, 21, 25–27), and diamond (Q²²⁴, Q²²⁷) (28–31) experimentally and theoretically. On the basis of theoretical prediction, the junction points (nodes) in the network phases could be coordinated with three, four, or six neighbors in three-dimensional space, resulting in the enhancement of packing frustration (31). Topologically, all these phases match the coordination number to neighbors (n = 3, 4, 6), showing no special case of quasicrystal. Accordingly, an order–order transition from double-diamond phase (DD, tetrapod) to double-gyroid phase (DG, trigonal planar network) has been observed (29). Yet, there is no DP phase being found in simple diblock systems except for liquid crystals (32, 33) or organic–inorganic nanocomposites from the mixtures of BCP with inorganic precursors (34, 35). Searching the rare occurrence of network phases and the corresponding phase transitions among phases will be essential to the demands for application by considering the deliberate structuring effects on aimed properties but the approaches remain challenging (8, 36–40). For instance, viewing the narrow window for network morphologies in diblock copolymer phase diagram, it demands harsh requirements for syntheses (2, 41). Recently, by taking advantage of using selective solvent for solution casting, it is feasible to acquire DG phase and even inverted DG phase from the self-assembly of lamellae (L)-forming polystyrene-block-polydimethylsiloxane (PS-b-PDMS) (42). Apart from that, a triclinic DG phase was recently discovered from the PS-b-PDMS which is commonly believed nonexisting in the conventional phase diagram (43). As a result, the phase diagram of BCPs with high interaction parameter is worthy of study for searching the metastable phases with unique network textures (44). Herein, we aim to acquire network phases from a simple diblock system by kinetically controlling the transformation mechanisms of self-assembly. As exemplified by using the PS-b-PDMS for solution casting, with the use of a PS-selective solvent (chloroform), a DP phase and a DD phase could be formed through controlled self-assembly, giving unique network phases simply from solution casting. Moreover, a DG phase can be also acquired from phase transformation. Consequently, a series of network phases with hexapod, tetrapod, and trigonal planar building units could be successfully obtained by using a single-composition L-forming PS-b-PDMS for self-assembly. The corresponding order–order transitions among these network phases examined by temperature-resolved in situ small-angle X-ray scattering (SAXS) combining with electron tomography results provide insights of network phase formation and the corresponding phase transformation mechanisms in the self-assembly of BCPs.